Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-12-28
2003-03-04
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C429S206000, C429S218200, C429S163000, C423S594120
Reexamination Certificate
active
06528209
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a active material for positive electrode for an alkaline secondary cell and a method for producing the same as well as an alkaline secondary cell using the above active material for positive electrode, especially a nickel-hydrogen secondary cell and a method for producing the same. More particularly, the present invention is concerned with to a active material for positive electrode exhibiting high utilization and a method for producing the same, and an alkaline secondary cell, especially a nickel-hydrogen secondary cell, which is advantageous not only in that the rise in cell internal pressure is suppressed and both the charge-discharge cycle characteristics and the overdischarge characteristics are improved, but also in that it has excellent storage properties.
2. Prior Art
As representative examples of alkaline secondary cells, there can be mentioned a nickel-hydrogen secondary cell and a nickel-cadmium secondary cell. These cells have incorporated thereinto, as a positive electrode, a nickel electrode comprised mainly of nickel hydroxide which is a active material for positive electrode.
As the nickel electrode, one which is of sinter-type and one which is of paste-type have conventionally been used.
Of these, the sinter-type positive electrode is generally produced as follows. For example, there can be mentioned a method in which nickel particles are sintered in a two-dimensional substrate comprised of a perforated steel or a nickel net to prepare a porous substrate, and the pores in the sinter of the above nickel particles in the prepared porous substrate is impregnated with an aqueous solution of a nickel salt, and further, the aqueous solution of a nickel salt is converted into nickel hydroxide which is a active material for positive electrode using an alkaline aqueous solution.
However, in the above production method, cumbersome treatments are needed, such as an impregnation of an aqueous solution of a nickel salt, a treatment using an alkaline aqueous solution and the like. Further, for forming a active material for positive electrode (nickel hydroxide) in the predetermined amount, it is required to repeat the above-mentioned treatments 4 to 10 times, so that a problem arises in that the production cost for a positive electrode is increased.
In addition, when the porosity of the sinter of nickel particles which are sintered in the above porous substrate exceeds 80%, it is difficult to secure the mechanical strength of the sinter. Therefore, the porosity of the sinter cannot be increased to 80% or more, resulting in a restriction on the method for increasing the capacity of the positive electrode by increasing the amount of the active material added.
For solving the above problems, studies have been made on the paste-type positive electrode produced by a method in which nickel hydroxide particles, particles of a conductor and a binder are kneaded together with water, to thereby prepare a paste for an active material, and a conductive core material (current collector) having a three-dimensional network structure, such as a spongy porous metal or a metal mat fiber, is filled with the above prepared paste, and then, dried and subjected to calendering treatment successively, and the paste-type positive electrode is being put into practical use.
This paste-type positive electrode has a porosity and an average pore size of the conductive core material larger than those in the above-mentioned sinter-type positive electrode. Therefore, the filling of the core material with the active material paste, i.e., active material, is easy, and further, the amount of the active material added can be increased. Thus, from the viewpoint of increasing the capacity of a cell, the paste-type positive electrode is advantageous, as compared to the sinter-type positive electrode.
However, on the other hand, the paste-type positive electrode has the following problem. Specifically, since the pore size of the conductive core material is large, the distance between the active material in the pores of the core material and the skeleton of the core material (current collector passage) is large. In addition, the active material per se is non-conductive. Therefore, the conductivity of the positive electrode itself becomes poor, and the active material at a position far from the skeleton of the core material does not relate to the cell reaction, so that a problem is encountered that the utilization of the active material is lowered.
For solving the above problem, with respect to the paste-type positive electrode, a number of studies have been made to increase the utilization of the active material by satisfactorily securing the electrical connection state between the active materials or the active materials and the conductive core material.
The most common method for solving the problem is a method in which, during the preparation of the positive electrode paste, particles of metallic Co or a cobalt (Co) compound, such as a Co hydroxide or a Co oxide, is added and mixed in the predetermined amount as a conductive auxiliary.
When a positive electrode having a core material filled with the above-mentioned positive electrode pate is incorporated into an alkaline secondary cell, any metallic Co or any Co compound contained in the positive electrode pate is dissolved in the alkaline electrolytic liquid as complex ions once, and the ions are distributed on the surfaces of the nickel hydroxide particles. Then, when the cell is subjected to initial charging, these complex ions are oxidized prior to nickel hydroxide and converted into cobalt oxyhydroxide which is conductive. The resultant cobalt oxyhydroxide is deposited between the nickel hydroxide particles which are an active material or between the active material layer and the skeleton of the conductive core material, to thereby form a conductive matrix in the positive electrode. As a result, both the conductivity between the nickel hydroxide particles as an active material and the conductivity between the active material and the conductive core material are improved, so that the utilization of the active material in the positive electrode is improved.
In such a case, the above-mentioned metallic Co and Co compound are unstable in air, and it is difficult to uniformly mix these particles with the nickel hydroxide particles. Therefore, for increasing the utilization of nickel hydroxide while securing the conductivity as a positive electrode, it is necessary that the amount of the Co compound added be about 10% by mass.
However, when the amount of the Co compound added is large, the relative content of the nickel hydroxide particles (active material) in the active material paste prepared is reduced. In addition, from the viewpoint of the cell design, there is a need to form a discharge reserve for the counter electrode (negative electrode), and this prevents the capacity of the positive electrode from being increased.
The positive electrode is generally accommodated in a cell casing together with a negative electrode, a separator and an alkaline electrolytic liquid.
Then, the assembled cell is subjected to aging treatment and initial charging treatment so as to subject the active material for positive electrode incorporated to activation treatment.
In this case, since the Co compound coexisting with the active material (nickel hydroxide particles) in the positive electrode is electrochemically reversible, the Co compound is discharged during standing of the cell for a long term or due to a microleak current by an electronic switch when the cell is mounted into an appliance, and loses the conductivity thereof. For this reason, although varies depending on the environmental conditions, the cell capacity is sometimes lowered by about 10 to 20% of the rated capacity.
In addition to the above-mentioned method for solving the problem, there is also a method in which, prior to the preparation of an active material paste, the surface of the active material (nickel hydroxide part
Bando Naomi
Miyamoto Kunihiko
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Toshiba Battery Co., Ltd.
Weiner Laura
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